The invention relates to a cathode for electrolytic processes, in particular to a cathode suitable for hydrogen evolution in an industrial electrolytic process. In the following, reference will be made to chlor-alkali electrolysis as a typical process of industrial electrolysis with cathodic hydrogen evolution, but the invention is not limited to a particular application.
In the industry of electrolytic processes, competitiveness is associated with several factors, the main of which is the reduction of energy consumption, directly correlated with the operative voltage. This justifies the many efforts directed to reduce the various components of the latter, for instance ohmic drops, which depend on process parameters such as temperature, electrolyte concentration and interelectrodic gap, besides anodic and cathodic overvoltage. For this reason, although some chemically resistant metallic materials deprived of catalytic activity—such as for instance carbon steels—may be used as hydrogen-evolving cathodes in various electrolytic processes, the use of electrodes activated with a catalytic coating has become more widespread with the purpose of decreasing the hydrogen cathodic overvoltage. Good results can thus be obtained by using metal substrates, for instance made of nickel, copper or steel, provided with ruthenium oxide or platinum-based catalytic coatings. Energy savings obtainable through the use of activated cathodes, in fact can sometimes compensate for the costs derived from the employment of precious metal-based catalysts. At any rate, the economic convenience of the use of activated cathodes basically depends on their operative lifetime. In order to compensate for the cost of installation of activated cathodic structures in a chlor-alkali cell, it is for instance necessary to guarantee their functioning for a period of time not lower than 2 or 3 years. Nevertheless, the vast majority of noble metal-based catalytic coatings suffer great damages following the occasional current reversals which can typically occur in case of malfunctioning of industrial plants: the passage of anodic current, even of limited duration, leads to a shifting of the potential to very high values, somehow giving rise to the dissolution of platinum or of ruthenium oxide. A partial solution of this problem was proposed in international patent application WO 2008/043766 incorporated herein in its entirety, disclosing a cathode obtained on a nickel substrate provided with a coating consisting of two distinct zones, one of which comprises palladium and optionally silver, with a protective function especially towards current reversal phenomena, and an activation zone comprising platinum and/or ruthenium, preferably mixed with a small content of rhodium, with a function of catalyst for cathodic hydrogen evolution. The increase in the tolerance to current reversal phenomena is presumably attributable to the role of palladium, which can form hydrides during the normal cathodic operation. During the reversals, hydrides would be ionised preventing the electrode potential from shifting to dangerous levels. Although the invention disclosed in WO 2008/043766 proves useful in extending the lifetime of activated cathodes in electrolysis processes, suitable performances are provided only by those formulations containing a significant amount of rhodium. In consideration of the very high price of rhodium and of the limited availability of this metal, this appears to be a strong limitation to the use of such kind of coatings.
There exists a need, then, for a new cathode composition for industrial electrolytic processes, in particular for electrolytic processes with cathodic evolution of hydrogen, characterised by a higher catalytic activity and by an equivalent or higher duration and tolerance to accidental current reversals in the usual operating conditions with respect to prior art formulations.